In many microelectronic, optoelectronic and electronic applications, it is advantageous to control the bonding energy in a composite substrate made of two bonded substrates.
The control of the bonding energy especially arises when the composite substrate is intended to be debonded. The debonding of the composite substrate is understood in this text as the separating of the bonded substrates without damaging them.
For example, it is interesting to be able to detach a semiconducting layer from a substrate in order to ultimately transfer it onto another support, the layer containing or not containing all or part of an electronic component.
To that end, the bonding energy shall be low enough to allow debonding of the composite substrate without damaging one or both of the substrates.
On the other hand, it is often required to have a bonding energy sufficiently high in order to avoid a separation of the bonded substrates before the desired moment of debonding.
One particularly advantageous application consists in using a semiconductor-on-insulator (SeOI) type of substrate capable of being removed from the insulator. An SeOI substrate such as this successively includes a so-called intermediate substrate, an insulating layer and a semiconducting layer. Removal from the insulator enables the semiconducting layer to be released and the intermediate substrate to be reused.
Various methods have been developed for transferring the semiconducting layer onto a final support, and for ultimately recovering the intermediate substrate.
The document WO 02/084722 describes the creation of an interface by bonding together a face of one wafer with a face of another wafer, comprising a step for pre-treating at least one of the two faces in order to control the degree of mechanical strength of the interface.
This treatment consists in controlling the roughness and/or hydrophilicity of at least one of the faces, the effect of which is to reduce the bonding energy of the debondable interface, and to thereby make it possible to facilitate the removal.
As a matter of fact, at a microscopic scale, roughening, as practiced in the prior art, creates surface cavities. Consequently, the actual contact surface area is smaller than the area of the bonding interface, which makes it possible to reduce the bonding energy. However, the roughening technique has a disadvantage in that the distribution of the cavities on the treated face is random and uneven. Furthermore, the shapes and dimensions (depth and area) of these cavities are not constant.
The result of this is that the process does not enable a predefined and reproducible bonding energy to be obtained.
Furthermore, this treatment is applied to the entire surface of the substrate and does not enable particular areas of the interface to be treated differently.
Another technique elaborated upon in the document WO 02/084721 proposes the creation of an interface with areas of different mechanical strength. This document provides for at least one first area having a reliable degree of mechanical strength to be surrounded by at least one second peripheral area having a higher degree of mechanical strength, in order prevent risks of delamination; in particular, an interface such as this can be in the form of parcels of reliable mechanical strength surrounded by areas of higher mechanical strength, each parcel corresponding to a component.
It is understood, therefore, that this process enables the creation of interfaces having bonding energies that are differentiated according to the parcels, but the parcels thus defined continue to be of considerable dimensions (from 1 micrometer to a few millimeters), with the result being that there is a risk of tearing during the removal operation.
The treatments described above are therefore imprecise because they do not enable the surface condition of the bonding interface, and consequently the bonding energy thereof, to be controlled at a sufficiently small scale.
Furthermore, they require perfectly uniform and planar interfaces for optimal bonding.
Another method for providing a debondable substrate wherein the bonding energy is controlled is disclosed in FR 2 783 235. This method consists in forming cavities on the surface of one of the substrates, so that the substrates are bonded only in the regions between the cavities. The bonding energy is controlled via the control of the total surface of the cavities. However, this method does not allow obtaining high bonding energy in the regions between the cavities, because the materials in contact are materials such as silicon, which have a relatively low bonding energy. This method thus provides a composite substrate with a limited bonding energy, which can be too low in some cases.